A. Field of the Invention
The invention relates to a method and an apparatus for controlling the mixture ratio of fuel and air in the combustile mixture supplied to an internal combustion engine. More particularly, the invention relates to a method and apparatus for controlling a combustible mixture on the basis of data from a .lambda. or oxygen sensor disposed in the exhaust system for monitoring the presence of free oxygen and hence the composition of the combustible mixture supplied to the engine. The oxygen sensor supplies an actual value signal to the control loop permitting the latter to regulate the mixture.
B. State-of-the-Art
Known in the art are systems which determine the duration of fuel injection control pulses for an internal combustion engine on the basis of an alternating signal from a .lambda. (oxygen) sensor located in the exhaust system of an internal combustion engine. The oxygen sensor signal is used by an appropriate mixture supply system for altering the fuel-air ratio. When an electric fuel injection system is used, the main variables for deriving the basic length of the fuel injection control pulses are the engine speed (rpm) and the air flow rate aspirated by the engine. The fuel injection control pulses are generated in synchronism with crankshaft revolutions or, alternatively, fuel may be injected continuously. The output signal from the .lambda.-sensor is compared with a threshold voltage by a comparator circuit. The resulting signal is integrated and fed back to the mixture preparation system. One of the problems encountered in the known .lambda.-sensor control systems is the operation in temperature domains where, due to the substantial cooling effect, the operation of the .lambda.-sensor is critical because its output signal is indistinct or varies drastically. A further problem occurs when the known systems are used in internal combustion engines which have more than a single exhaust channel so that a single .lambda.-sensor is able to monitor the exhaust gas composition and hence the fuel mixture composition only for one-half of the engine, for example.